In railway vehicles, various items of information are displayed in the cabin in order to be monitored by the driver.
From the various items of displayed data, some are more particularly sensitive since they lead to control actions on the part of the driver.
This is the case, for example, for displaying the instantaneous speed of the vehicle.
The driver regularly takes this displayed information into account to correct the speed of the vehicle in order to comply with the speed limits imposed along the railway track.
It is therefore imperative to provide a reliable display for these sensitive items of data.
Generally, display devices located in the cabin show the information in the form of an image displayed on a screen. In this instance, the display device is controlled by a video controller. The video controller receives, from a main controller, a series of graphic orders which correspond to the image to be displayed; the video controller processes this series of graphic orders to generate a “bit-map” or “a matrix of pixels”, which is a point by point representation of the image to be displayed; it stores this bit-map in a dedicated video memory; and it encodes this bit-map in an appropriate manner in order to transmit it to the display device in order to actually display the image on the screen.
Document FR 2 868 193 discloses a display architecture which comprises a means which is for verifying the integrity of the displaying and which is capable of reading the content of the video memory of the video controller and comparing the content read with a reference value.
According to this architecture, the verification allows potential malfunctions to be detected upstream of the video memory of the video controller.
However, a number of risks of error may affect the video controller itself, in particular its manner of encoding the bit-map in order to transmit it to the display device, and the integrity of the connection between the output interface of the video controller and the input interface of the display device. It should be noted that, in some arrangements, the video controller and the display device may be spaced-apart from each other, for example, by 10 meters or more.
Furthermore, video controller manufacturers do not willingly describe the internal architecture of their hardware and how to access the contents of the memory.
Document WO 2007/033902 describes a display architecture in which, for each value of information to be displayed, a specific pictogram is introduced into a specific zone of the image to be displayed. By analyzing this particular zone of the image, it is possible to detect the pictogram and derive from it the value of information which it encodes. Comparing this derived value with the reference value allows any inconsistencies to be detected which may be symptomatic of a malfunction in the displaying chain. However, in addition to the fact that this technique overloads the image, it has the significant disadvantage of only verifying with certainty the zone where the additional pictogram is displayed, without truly verifying the zone where the information used is displayed.
Document FR 2788365 describes a liquid crystal display device comprising a plurality of elements which are independently controlled, of the “7-segments” type. It also comprises means for rereading the state of each element, which allows the verification that each element is really in the state corresponding to the control instruction applied to it. Of course, this technique cannot be transposed to displays of pixel matrixes of great size since it would require control circuits for each pixel, which would make the overall device too complex and costly.